A conventional multitube reactor is equipped with a plurality of reaction tubes having a catalyst packed therein and a plurality of baffles inside a shell for feeding and circulating inside the shell a fluid for heat removal (hereinafter, referred to as “heat medium”) introduced into the shell. A raw material gas fed inside the reaction tubes reacts in the presence of the catalyst inside the reaction tubes, to thereby generate heat of reaction. The heat of reaction is removed by a heat medium circulating inside the shell.
When differences of inner volumes among the plurality of reaction tubes equipped inside the shell is large, amounts of the catalyst packed inside the reaction tubes are irregular and a scatter arises. As a result, a flow rate of the raw material gas fed or a retention time differs among the reaction tubes, thereby becoming a factor causing yield reduction of a target product and reduced catalyst life. Further, a localized abnormal high-temperature site (hot spot) may form in the reaction tubes provoking a reaction out of control, thereby causing a problem of inhibiting a continuous operation.
Further, in a multitube reactor provided with the baffles, the heat medium does not flow at all in a portion where the baffles and the reaction tubes are fixed to each other when the baffles and the reaction tubes are fixed through welding, flanges, or the like. A reactor in which outer walls of the reaction tubes and the baffle are not fixed also exists, but the amount of the heat medium flowing through this clearance is limited. The following problems arise in a vapor phase catalytic oxidation method using a fixed bed multitube heat-exchanger type reactor as described above.
There is a state of poor heat removal in the reaction tubes in a portion where flow of the heat medium is insufficient inside the shell. A localized abnormal high-temperature zone (hot spot) may form in the reaction tubes which are in a state of poor heat removal, possibly resulting in a reaction out of control. Further, a reaction may not become out of control, but problems arise including ease of clogging the reaction tubes, yield reduction of the reaction product gas, deterioration of the catalyst life, and inhibition of a stable operation for a long period of time.
Many methods of suppressing hot spot formation have been proposed for the multitube reactor used in a vapor phase catalytic oxidation reaction. For example, JP 08-092147 A discloses a method of providing uniform heat medium temperature by: setting a flow direction of a reactant gas guided to a reactor and that of the heat medium inside a shell in a countercurrent; allowing the heat medium to flow further upward in a meandering way using baffles; and adjusting temperature differences of the heat medium from an inlet of the reactor to an outlet thereof within 2 to 10° C. or less.
The multitube reactor generally consists of a plurality of tubes (bundle) arranged vertically, and thus a process fluid flow can be upflow or downflow by allowing a process fluid to flow from an upper portion or lower portion of the reactor. The heat medium can also be fed to the shell from the upper portion or lower portion thereof.
Therefore, the multitube reactor is classified into two types similar to a general shell and tube heat exchanger: a concurrent type allowing the process fluid and the heat medium to flow in the same direction; and a countercurrent type allowing the process fluid and the heat medium to flow in opposite directions.
Further, the multitube reactor may be classified into the following types considering the directions of the fluids: 1) a concurrent type of downflow process fluid/downflow heat medium; 2) a concurrent type of upflow process fluid/upflow heat medium; 3) a countercurrent type of upflow process fluid/downflow heat medium; and 4) a countercurrent type of downflow process fluid/upflow heat medium.
Proposed in JP 2000-093784 A is a method of suppressing hot spot formation by: allowing a raw material gas and a heat medium to flow in downward concurrent; and preventing a gas reservoir free of the heat medium. Further, the method allows an exchange of a catalyst in a vicinity of a catalyst layer inlet alone where most easily deteriorates by: feeding the raw material gas from an upper portion of a reactor; and allowing the raw material gas to flow downward inside the catalyst layer of reaction tubes.
However, the heat medium and the process fluid move in a concurrent according to the method, and gas temperature in an outlet portion of the reactor increases. Thus, the method has a fault that high concentration of a product (meth)acrolein easily causes an autooxidation reaction (autolysis reaction).
Further, with respect to the upflow, in a method of allowing the process fluid and the heat medium to flow in a concurrent, that is, in the same direction, heat medium temperature increases with heat of reaction. Thus, high temperature at a process outlet causes autooxidation at the reactor outlet easily. The autooxidation reaction results in problems of a combustion reaction of the product, equipment breakdown due to temperature increase, and yield reduction.
Proposed is a method of preventing autooxidation for a purpose of preventing temperature increase, by providing a cooling zone or heat exchanger in a downstream of a reaction portion for decreasing gas temperature. However, in a concurrent, heat medium temperature in the vicinity of the reactor outlet and process gas temperature in an outlet portion are high. Thus, an amount of heat removal becomes large and a cooling portion (cooling zone and heat exchanger) enlarges, thereby becoming disadvantageous in point of cost.
Further, even if a significant autooxidation reaction is not caused, an autooxidation reaction is caused by a part of a product, which arises a problem of yield reduction of a target product as a whole.
Further, in a shell-tube type reactor circulating a heat medium which is solid at normal temperature, there is a necessary to maintain the heat medium at temperature of the solidifying point or above to ensure fluidity thereof for circulating the heat medium inside the reactor.
JP 2001-310123 A discloses a reactor start up method for a multitube reactor having reaction tubes, an introducing port of a fluid flowing outside reaction tubes, and a discharging port thereof for removing heat generated inside the reaction tubes, the method being characterized by including: heating reaction tubes by introducing a gas having temperature of 100 to 400° C. in the reaction tubes; and circulating a heated heat medium through the outside of the reaction tubes. Further, a gas not providing an effect when being mixed with a catalyst packed in the reaction tubes or with a raw material gas (such as air) is selected as the gas introduced to the reaction tubes.
However, a large volume of a high temperature gas is introduced to the reaction tubes according to the above-mentioned method, thereby changing an oxidation state of the catalyst. Therefore, catalytic activity and selectivity may be affected, possibly resulting in yield reduction or reduced catalyst life.